As the speed and power demands of microprocessors have grown, the use of heatsinks has become common. Heatsinks are devices that are disposed upon a microprocessor, in order to transfer heat away from the microprocessor. Like the microprocessors themselves, heatsinks may be found in a variety of shapes and sizes. As the amount of heat produced by a microprocessor increases, the heatsinks typically grow in size and weight.
When two smooth surfaces, such as a microprocessor and a heatsink, are placed in contact with one another, air voids between the surfaces impede effective heat flow. Thermal interface materials (TIMs), usually made up of viscous materials like grease or other thermal phase change materials, fill in the spaces between the two surfaces and are used to improve the transfer of heat between the devices. The thermal interface materials conform to the otherwise rough and uneven mating surface of the smooth objects, providing a substantially flat interface that ensures close contact between all portions of the microprocessor and heatsink.
Most heatsinks are coupled to the microprocessor using a thermal interface material. An assembly including thermal interface material between the microprocessor and the heatsink is more effective at transferring heat to the heatsink than simply coupling the heatsink directly to the microprocessor. By properly loading the assembly, a uniform thermal interface between the heatsink and the microprocessor, and, thus, better heat transfer, can be achieved. The thermal interface material may be designed for loads of 10-90 psi (pounds per square inch) or higher.
Some heatsinks are clipped upon a socket of the microprocessor and are mounted to the system board. The clipped-on heatsinks typically weigh about 200 grams. The spring clips provide two functions: they hold the heatsink in place and they load the assembly so that a uniform thermal interface between the heatsink and the processor is formed.
Heavier heatsinks may be connected to the processor differently. Although still mounted to the system board, a 400-gram heatsink, for example, would likely be mounted through holes in the system board to a chassis. Despite the sturdier connection to the microprocessor, these heatsinks nevertheless fail shock and vibration tests, particularly when forces perpendicular to the thermal interface material are applied to the assembly.
In some systems, heatsinks are constructed using copper, rather than aluminum, due to the better heat conduction properties of copper. Since copper is a heavier material than aluminum, relatively heavier heatsinks are to be expected with this material. Heatsinks weighing 1000 grams or more have become common. With this significant weight, meeting shock and vibration testing requirements becomes increasingly difficult.
The printed circuit board upon which the microprocessor and heatsink are retained is designed to be somewhat rigid. Some flexion of the printed circuit board may occur, but component failure is more likely when the board is flexed. In addition to the impedance, trace width, and other considerations of the system design, the mass of any component placed upon the printed circuit board can impair signal integrity. Shock and vibration testing will likely fail if the weight of one or more components on the printed circuit board exceeds board specifications.
The mass of the heatsink can also damage the processor, the processor pins, or the socket. Like the printed circuit board, these components are rated to have predefined load limits. Where the heatsink is spring-clipped, the excess mass of the heatsink overloads the springs, causing excessive deflection to the printed circuit board, the processor, and/or the socket.
During shock and vibration testing, a system board is generally tested in three dimensions. Thus, the effect upward and downward forces in the X-axis, the Y-axis, and the Z-axis (perpendicular to the system board) are measured. Although failures in testing the other axes also occur, during the Z-axis testing in particular, the weight of the heatsink can adversely affect system components, such as the system board, the processor socket, and the processor itself.
In shock and vibration tests in which the force applied to the system is perpendicular to the thermal interface material (Z-axis testing), the heatsink is actually pulled away from the processor, due to the presence of the TIM. The grease-like thermal interface material covers the surface area of the processor. As the heatsink is pulled quickly during the perpendicular load testing, the grease causes a suctioning effect, and the load ends up being transferred to the processor pins. The force may quickly pull the processor from its socket, damaging the processor pins, the socket, and/or the processor.
Thus, there is a continuing need to provide a heatsink retention mechanism that supports high-mass heatsinks and meets typical shock and vibration testing criteria.